Spatio-temporal ultrafast pulse shaping at the femtosecond-nanometer scale.

Optical pulse shaping is a fundamental tool for coherent control of the light-matter interaction. While such control enables the measurement of ultrafast temporal dynamics, simultaneous spatiotemporal control is required for studying non-local ultrafast charge dynamics at the nanoscale. However, obtaining accurate spatial control at a sub-wavelength resolution with conventional optical elements poses significant difficulty. Here, we use the spatiotemporal coupling naturally arising in a spatial light modulator based pulse shaping apparatus to achieve accurate control with femto-nano spatiotemporal resolution. We experimentally demonstrate spatial steering at the sub-micron scale of second harmonic generation from nanostructures. In addition, we apply an absolute-value spectral phase to achieve controlled double pulses for nanoscale excitation. We introduce a novel, to the best of our knowledge, scheme for accurate tunable spatiotemporal pump-probe experiments. This method offers rich insight into materials with ultrafast transport phenomena at the femtosecond-nanometer regimes.

Adiabatic four-wave mixing frequency conversion.

We introduce the concept of adiabatic four-wave mixing (AFMW) frequency conversion in cubic nonlinear media through an analogy to dynamics in quantum two-level systems. Rapid adiabatic passage in four-wave mixing enables coherent near-100% photon number down-conversion or up-conversion over a bandwidth much larger than ordinary phase-matching bandwidths, overcoming the normal efficiency-bandwidth trade-off. We develop numerical methods to simulate AFWM pulse propagation in silicon photonics and fiber platforms as examples. First, we show that with a longitudinally varying silicon waveguide structure, a bandwidth of 70 nm centered at 1820 nm can be generated with 90% photon number conversion. Second, we predict the broadband generation of nanojoule energy, 4.2-5.2 µm mid-infrared light in a short, linearly tapered fluoride step-index fiber. We expect the AFWM concept to be broadly applicable to cubic nonlinear platforms, for applications as diverse as bright ultrafast light pulse generation, sensing, and conversion between telecommunications bands.

Pulse shaping of broadband adiabatic SHG from a Ti-sapphire oscillator.

We experimentally demonstrate an efficient broadband second-harmonic generation (SHG) process with a tunable mode-locked Ti:sapphire oscillator. We have achieved a robust broadband and efficient flat conversion of more than 35 nm wavelength by designing an adiabatic aperiodically poled potassium titanyl phosphate crystal. Moreover, we have shown that with such efficient flat conversion, we can shape and control broadband second-harmonic pulses. More specifically, we assign a spectral phase of absolute value and π-step, which allows wavelength tunable intense pump-probe and amplitude modulation of the broadband second-harmonic output. Such spectral phases serve as a proof of concept for other pulse-shaping applications for nonlinear spectroscopy and imaging.